1. Field of the Invention
The present invention relates generally to a switch circuit, and more particularly, to a transistor single-pole-single-throw circuit device.
2. Description of Related Art
A metal-oxide-semiconductor (MOS) field-effect transistor (MOSFET) is a type of FETs widely applied in electronic devices, such as computers and communication apparatuses. Please refer to FIG. 1. A typical MOS 10 comprises at least three ports: a gate 11, a source 12, and a drain 13. The gate 11 comprises a metal layer 111, an oxide layer 112 and a semiconductor layer 113 stacked in order and has a structure similar to a capacitor. Thus, the MOSFET 10 is named a metal-oxide-semiconductor field-effect transistor in accordance with its structure.
Before the field-effect transistor operates, no external voltage is applied between the gate 11 and the source 12, and no current flows from the source 12 to the drain 13. When a voltage difference VGS is applied between the gate 11 and the source 12 of the MOSFET 10 and the voltage difference VGS reaches to a predetermined threshold voltage VT, induction charge emerges on an interface between the oxide layer 112 and the semiconductor layer 113, generating a conduction electron layer of an inversion channel.
The conduction electron layer allows currents to flow between the drain 113 and the source 112. Currents flowing through the inversion channel of the MOSFET 10 vary in accordance with the voltage applied to the gate 111. When VGS>VT, a voltage difference (VGS-VT) between the voltage applied to the gate 111 and the threshold voltage VT provides the conduction electron layer with a potential energy. Therefore, the electron density of the conduction electron layer is approximately directly proportional to (VGS-VT), and an on-resistance Ron of the conduction electron layer is inversely proportional to (VGS-VT). Further, since the on-resistance Ron is related to the capacitance Cox of the oxide layer 112, the mobility μ n of electrons in the conductive channel, the width W of the transistor and the length L of the transistor, the on-resistance Ron of the field-effect transistor can be expressed as:
      R    on    =      1                  μ        n            ⁢              C        ox            ⁢              W        L            ⁢              (                              V            GS                    -                      V            T                          )            
It can be seen from the above equation that the resistance of the field-effect transistor can be varied by adjusting the gate voltage VGS such that the channel resistance during turn-on is reduced to a value approaching zero or the resistance during turn-off is increased to another value approaching infinity. Therefore, the field-effect transistor can be designed as a switch for analog signals.
A field-effect transistor switch can be applied to a variety of circuits, such as a sample-and-hold circuit, a chopper circuit, an analog-to-digital converter, and a switch-capacitor filter.
The performance of a field-effect transistor switch has a connection with the capacitance of the gate oxide layer of the field-effect transistor, because the gate oxide layer of the field-effect transistor become thinner and thinner as the size of the field-effect transistor becomes smaller so that more field-effect transistors can be installed on a single chip. Further, because the conductive electrons move along the channel at an interface between the oxide layer and the semiconductor, the interface between the oxide layer and the semiconductor has to be fabricated as smooth as possible. Moreover, because a substrate of the semiconductor has a characteristic of low resistance, parasitic capacitance effects and leakage currents emerge between metal wires connecting the transistors by the smooth turn-on interface and the low-resistance substrate of the semiconductor, affecting the switching speed and operating efficiency of the field-effect transistor switch.
In the main-stream semiconductor manufacturing process, a gate oxide layer can have a thickness as small as 1.2 nanometer, a thickness of five stacked atoms. Under such a size, every physical phenomenon obeys the quantum physics such that a tunneling effect can no longer be ignored. Moreover, the tunneling effect enables electrons to have the chance to pass over the potential barrier formed by the oxide layer to generate leakage currents, which are one of the most dominant sources for a modern integrated circuit chip to consume power. Further, the parasitic capacitance is slight capacitance actually existing between conductive wires and a silicon substrate on the integrated circuit. When the signal frequency becomes high, the loss of the small signal of the circuit due to the parasitic capacitance effect is too large to be ignored.
Please refer to FIG. 2, which is a circuit diagram using a field-effect transistor as a single-pole-single-throw switch according to the prior art. A conventional single-pole-single-throw circuit 20 at least comprises a switch field-effect transistor 21 in charge of a switching function of the circuit, and an isolation field-effect transistor 22 for improving the isolation when the circuit is turned off. The properties of a switch circuit require that it is better for the switch field-effect transistor 21 to have a small on-resistance Ron, in order to reduce the signal loss due to too large a resistance when the circuit is turned on. When the switch field-effect transistor 21 is turned off, it is better for the isolation field effect transistor 22 to have a small on-resistance Ron, in order to for the switch circuit to have a well isolation when turned off. As implied in the above equation of the on-resistance Ron of the field-effect transistor, when the substrate is limited to be manufactured by a certain material and the external applied voltage is specified specifically, the on-resistances of the above two transistors can be adjusted by changing the width and the length of the transistors. However, the length of a transistor is limited by a semiconductor manufacturing process and is constant, it is a common way to change the on-resistance by adjusting the width of the field-effect transistor.
In summary, in order to ensure the quality of signals passing through the switch, the conventional field-effect transistor single-pole-single-throw selects a switch transistor with a larger width, so as to reduce the on-resistance of the switch transistor and the loss of signals over transmission, to ensure the quality of the transistor switch. However, since the width of the transistor is increased, the area of the oxide layer capacitor included in the transistor is increased accordingly, and the parasitic capacitance of the transistor is increased accordingly.
Therefore, how to provide a field-effect transistor single-pole-single-throw switch circuit device, which prevents signal loss due to the emerged parasitic capacitance when selecting a switch transistor having a larger width, is becoming an urgent task in the art.